The present invention relates to a subscriber line interface circuit for a telephone system, and more particularly to a subscriber line interface circuit and method of detecting a ground key and/or a fault in which the ground key or fault is indicated by comparing tip and ring DC currents to a reference current.
A telephone subscriber line interface circuit (SLIC) connects a balanced two wire transmission path (the path to and from the subscriber telephone handset) with an unbalanced four wire transmission path (the path to and from the telephone central station). SLICs perform various functions, including battery feed, overvoltage protection, ringing, signaling, hybrid, and timing. SLIC operation is known and need not be considered in detail. By way of brief explanatory example, and with reference to FIG. 1, a current mode SLIC 10 (a SLIC that processes the currents related to the signals in the transmission paths, rather than the voltages) uses current information available in the tip and ring voltage sensing resistors RS1 and RS2 at the two wire side of the SLIC. The current information is provided to a hybrid circuit 12 that detects incoming signals and sends them in the right direction. The hybrid circuit 12, in combination with external circuitry 14, provides appropriate amplification, far end echo cancellation, and impedance matching.
Some telephone systems ground the ring side of the telephone line to signal the operator, this signal being denominated a ground key. In such systems, additional circuitry may be provided to indicate a ground key. However, such additional circuitry adds complexity and provides further opportunities for connections to fail. Accordingly, it is desirable to reduce the amount of additional circuitry provided for ground key detection.
A SLIC also desirably is able to protect itself from faults, such as accidental grounding by linemen, cut or downed power lines, etc. If the SLIC does not detect the fault, it will attempt to drive the short and may overheat.
Accordingly, it is an object of the present invention to provide a novel ground key detector for a SLIC and method of detecting a ground key that obviates the problems of the prior art.
It is a further object of the present invention to provide a novel ground key detector for a SLIC and method of detecting a ground key in which the ground key is indicated by comparing tip and ring currents to a reference current.
It is yet a further object of the present invention to provide a novel SLIC with an integral ground key detector.
It is another object of the present invention to provide a novel fault detector for a SLIC and method of detecting a fault in which the fault is indicated by comparing tip and ring currents to a reference current.
It is still another object of the present invention to provide a ground key and fault detect circuit for a SLIC in which the ground key and/or fault are indicated by comparing tip and ring DC currents to a reference current.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.